The present invention relates to plasma-processing sources, systems, and methods, and particularly to inductively-coupled plasma source architectures.
Standing waves unavoidably develop on inductively coupled plasma (ICP) sources because they are mismatched transmission line systems. In addition, the source electrical properties are coupled to those of the plasma, since the source-plasma system behaves similar to a transformer. As a result, the source input impedance and the RF wavelength on the source can be substantially affected by the very plasma conditions it generates. This can also influence the symmetry of the source electromagnetic fields and plasma generation, which in turn will influence the uniformity of the plasma and the ion flux to the wafer surface.
Inductively coupled plasmas behave like an air-core transformer with the inductive source-coil as the primary circuit and the plasma as the secondary (single current loop) circuit. The coil impedance is coupled to that of the plasma and changes with the plasma conductivity, which determines the plasma resistance and reactance, causing changes in the electrical characteristics of the inductive coil. The effect of plasma loading on the coil""s voltage, current and phase shift in argon discharges has been studied using transformer theory. (See Piejak 1992, Godyak 1994, Gudmundsson 1997, Gudmundsson 1998, Fayoumi 1997, and Fayoumi 1998, cited below.) Changes in the electrical characteristics of the coil due to plasma loading affect its electromagnetic fields, which largely determine the plasma generation symmetry and process uniformity. Understanding the interaction between the coil""s fields and the plasma is essential for inductive source design and scaling in order to optimize plasma process uniformity. Gudmundsson et al. (Gudmundsson 1998) modeled and measured the changes in the source-coil""s resistance and reactance at 13.56 MHz caused by plasma loading. El-Fayoumi et al. (Fayoumi 1997, Fayoumi 1998) measured the current induced in argon plasmas generated with a low frequency ICP source-coil. They calculated the plasma resistance and inductance from the induced plasma current and studied their effects on the coupling constant with the coil and its electrical properties.
Most studies have considered the coil""s voltage and current to be spatially averaged and did not take into account the effect of plasma loading on the standing wave pattern that unavoidably develops on ICP sources. Transmission line properties of an ICP source result in voltage and current standing waves along its length. The variations in current with position lead to asymmetries in the induced electro-magnetic fields, which in turn can lead to asymmetries in the power deposition, plasma generation and non-uniformity in the processing. (Jaeger 1995, Kushner 1996, Lamm 1997) A three-dimensional model by Kushner et al. (Kushner 1996) showed that the transmission line properties of the coil should influence the power deposition symmetry as well as the ion flux uniformity to the wafer surface. They examined the effect of capacitive termination impedance and coil geometry on the standing wave pattern and power deposition symmetry. In a related study, Lamm 1997, an ICP was modeled as a uniform transmission line system. Lamm made measurements of the standing wave for different source geometries and powers from which he derived analytical expressions for the spatial variations of the voltage and current along the coil length. More recently, Wu et al. (Wu 2000) investigated the influence of source configuration and standing wave effects on argon discharge density profiles generated with a large area ICP source. They modeled the inductive discharge as a lossy transmission line system and applied a transformer model to study the electrical properties of the system. In addition to a matching network, they used a tuning network to launch a traveling wave or a wave with a desired standing wave ratio along the source length. Their experiments showed that the source configuration and standing wave ratio could strongly influence the plasma density profile. Changes in the standing wave pattern on a new ICP source design caused by changes in plasma loading for argon and chlorine discharges have been reported recently by the inventors (Khater 2000, Khater 2001). The voltage and current variations along the coil""s length, as well as the phase difference between them, are determined by the coil""s characteristic impedance. Since plasma loading changes the coil""s characteristic impedance, the standing wave pattern will also change depending on the plasma conditions. As a result, the plasma generation symmetry and uniformity for a fixed ICP source geometry changes as the plasma conditions are varied. This effect should be considered in the design of ICP sources as they are scaled to large sizes for processing large area substrates.
A Faraday shield can be used to minimize these deleterious effects if properly designed and positioned. To date, Faraday shields have been used simply to decrease capacitive coupling between the source and the plasma and reduce sputtering of the dielectric window. Faraday shields have been used in this fashion for at least several decades. A dielectric spacer is placed between the source and the Faraday shield to provide electrical insulation. In most cases, air is chosen to be the dielectric because air has the lowest relative permittivity and results in the smallest standing-wave variation on the source. The present application teaches that a xe2x80x9csource-coil/dielectric spacer/Faraday shieldxe2x80x9d assembly acts as a transmission line with a nearly fixed characteristic impedance and standing wave pattern on the source-coil. In this manner, the source impedance is made stable regardless of plasma conditions since the Faraday shield decouples the source-coil electrical properties from those of the plasma. The key to designing this ICP source-coil/dielectric spacer/Faraday shield assembly is to ensure that the impedance between the shield and ICP source-coil dominates over the impedance between the ICP source-coil and plasma. When this is the case, changes in the plasma characteristics can cause little or no variation in the total ICP source-coil impedance and therefore become negligible. As a result, the standing wave pattern on the ICP source-coil becomes constant, as does the input impedance and plasma generation symmetry.
Such an assembly has important implications for plasma system design and optimization. For example, the use of this type assembly allows any ICP source to be impedance matched by a nearly fixed matching circuit. The possibility of a fixed matching condition will allow much simpler plasma control in addition to easily allowing for pulsed plasma processing with very little reflected power. This has been demonstrated experimentally (Khater 2001). In addition, once the source geometry is optimized for symmetric electromagnetic fields and plasma uniformity with a fixed standing wave pattern, it should stay uniform regardless of the plasma conditions. Optimizing the structure of the dielectric spacer (materials, shape) and Faraday shield structure in addition to the source-coil geometry is important in optimizing the electromagnetic field symmetry.
Finally, a calibrated aperture in the center, at the edge, or at some other location in the Faraday shield can be designed to allow a small amount of capacitive coupling to the plasma for striking the discharge. Once a high-density plasma forms, it will expel this capacitive coupled field and result in an inductively coupled plasma. Consequently, the source will both strike reliably and result in very little window sputtering or other deleterious effects. In addition, the Faraday shield/dielectric spacer/source-coil assembly will still prevent the plasma from changing the source-coil standing wave pattern, input impedance, and fields symmetries.
Transmission Line Based Inductively Coupled Plasma Source with Stable Impedance
The present inventors have realized that the Faraday shield/dielectric spacer/source-coil assembly provides a fundamental change in the electrical characteristics of the coil which drives the plasma, and that this change permits new techniques for operating an inductively-coupled plasma reactor. Without a Faraday shield, the RF behavior of the coil is determined by the state of the chamber""s interior, which varies dynamically. The complex impedance of the coil changes dramatically when the plasma is ignited, but also is dependent on other factors, such as pressure, which affect the electron density of the plasma. Since the coil is electrically coupled to the plasma, changes in the electron density of the plasma also change the complex impedance of the coil.
With the Faraday shield, capacitive coupling between the coil and the plasma is largely removed. The present inventors have realized that this makes the coil""s complex impedance much more independent of changes in the electron density of the plasma, and that this is very beneficial in optimizing the uniformity and controllability of the plasma source. Conventional ICP systems must allow for a large shift in complex impedance. One result of this is that conventional systems must use automatic matching networks which can adapt to large changes in the magnitude of impedance, e.g. over a range of ten to one.
The electrical behavior of an inductive source-coil is that of a transmission line, which forms the primary of a transformer. The transformer""s secondary is the loop of current that flows in the plasma. Conventional wisdom is that current nodes must generally be avoided on the source-coil. A current node on a transmission line will result in the appearance of voltage antinodes, i.e. locations where the voltage has a much larger magnitude than at other parts of the transmission line. This can result in increased erosion of the dielectric shield at such points. (A xe2x80x9cnode,xe2x80x9d analogously, is a location where the current or voltage is lower than at adjacent positions.) Moreover, the current distribution will be very non-uniform under such conditions, and this can result in hot spots, at unpredictable locations in the plasma, which cause non-uniformities in the wafer processing. The present inventors have realized that the use of a three-dimensional source-coil design coupled with the Faraday shield can allow one to circumvent conventional wisdom. One can place current nodes on the source-coil without causing hot spots and still produce symmetric, uniform plasma. The ability to have current nodes on the source-coil, however, allows one to produce uniform plasma over much larger areas.
In one class of embodiments, the decoupling effect of the Faraday shield is used to permit operation of the coil in resonant or near-resonant conditions. Since the coil is decoupled from the variations in the plasma electron density, the location of voltage and current antinodes is less likely to shift unpredictably. Moreover, since the current distribution in the coil is now more predictable, the geometry of the coil can be modified to increase the uniformity of power deposited into the plasma.
In a further class of embodiments this idea is taken even farther, and the coil, supporting dielectric, and Faraday shield are all jointly optimized for plasma uniformity.